No benefits shall be assessed in respect of the cost of any improvement or works in so far as such improvement or works extends beyond the line of the whole property assessed, but the cost thereof shall be assessed upon and paid by the respective contracting municipalities in accordance with the apportionment of the percentage of the costs, damages and expenses of the improvement provided to be borne and paid by them respectively in the contract made and entered into between them, as authorized by ordinance as aforesaid.